The present invention relates to an impeller for stirring sterile liquids.
More particularly, it relates to an impeller which has an impeller head with stirring plates and an opening arranged below for receiving a pin in a central hollow chamber, while the impeller is driven inductively or magnetically in a contactless manner and the central hollow chamber has at least one conduit connectable with the outer surface.
Such an impeller is disclosed for example in the U.S. Pat. No. 4,993,841 and the European patent document EP-A1 0 399 972. The impeller head has a plurality of radially outwardly extending stirring plates, the conduits open behind them as considered in the rotary direction on the outer surface, and the outer surface is connected with a central hollow chamber. Since during the operation, a negative pressure zone is formed behind the stirring plates, liquid is aspirated through these openings from the inner hollow chamber which is completed by the lower opening for receiving the drive pin. In this manner, during the operation, a stream flows through the inner hollow chamber of the impeller. During cleaning of the container with corresponding sterilizing liquids, it can also reach the interior of the impeller so that dead corners are avoided in which the process liquid can remain after withdrawal of the charge.
The above-described impeller has, however, the disadvantage that the self-cleaning effect does not act for low filling heights when the liquid no longer flows through the conduit connected with the highest opening of the hollow chamber with the outer surface, but instead is connected with the atmosphere. Due to the flow field which is formed during the operation, this condition occurs at the filling height where the fluid level in the static case completely covers the impeller.